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Not All Those Who Wander are Lost
By: Kelly A. Salzo
Long before the colonization of New England, Brook Trout occupied most of the watersheds throughout the northeastern corner of North America, with a natural range spanning from eastern Canada to as far south as Georgia. With the arrival of European settlers and their quest for industrialization, these once abundant, pristine waters became heavily polluted and obstructed by dams. The Post-Industrial Era only exacerbated the situation further, triggering a surge in the construction of roads, sidewalks, and other impervious surfaces that send contaminated rainwater straight to waterways. Now with our awareness of anthropogenic climate change, scientists report that rising stream temperatures are reaching a tipping point. As streams undergo such rapid and dramatic changes, countless Brook Trout are lost and those that remain must retreat to cooler waters in smaller headwater streams, forming small, isolated groups. Unfortunately, the potential for gene flow is severely limited in secluded populations, often leading to reduced genetic diversity, which threatens the sustainability of local populations through inbreeding depression. However, with all of this history stacking up against the trout, how do they continue to persist in some surprising places?
Laying the groundwork, former doctoral student Dr. Yoichiro Kanno (now with Colorado State University) teamed up with our own Dr. Jason Vokoun and Dr. Benjamin Letcher of the USGS S.O. Conte Anadromous Fish Research Center to investigate the population structure of Brook Trout across two headwater channel networks in Connecticut. Their research revealed heavily-structured population genetics within even small headwater watersheds. Each stream section that supports breeding was found to contain its own genetically-related group of fish sub-divided from the population. Similar observations have been reported throughout the region.
Along the way, Kanno also detected some interesting movements. Typically characterized as homebodies, Brook Trout rarely move from a single 50-meter stretch for months at a time. However, while virtually all trout he observed traveled upstream to their fall breeding grounds as expected, to his surprise a few headed downstream into larger rivers too warm to support trout in the summer months, but cool enough come mid-autumn. Why did these trout stray from the group? Were they lost? Or could they be venturing off into other headwaters?
The answer to this question may come straight from a fantasy novel. Just as Middle-Earth’s rangers wander as mere vagabonds to defend their lands from enemy forces, a select few Brook Trout wander from their native streams and shake up the gene pools to combat inbreeding and better their species’ chances of survival. While the motives of a wandering trout remain unknown, and chance and mistake is not off the table, an understanding of how these individuals are able to migrate can help shed light on ways that conservation and fisheries management can further increase the mobility of these important individuals.
Enter doctoral student Lucas Nathan. Under the advisement of Dr. Vokoun and Dr. Amy Welsh of West Virginia University, his current research involves utilizing genetic information from sampled trout to investigate the relatedness of headwater Brook Trout populations. Complementing Kanno’s results, Nathan’s research will involve larger spatial scales to discover genetic connections among headwater populations across the state of Connecticut.
Using genetics as a tool for conservation efforts is a relatively new and exciting concept. While traditional sampling may track the movement of fish over the course of a week or month, genetic sampling can give historical insight on the movement of individuals over the course of years or even decades. And unlike classic mark-recapture, “We just need to capture them once. Then by looking at population based metrics of genetic similarity, we can infer where and how often individuals are migrating and interbreeding among populations,” Nathan explains.
Since last winter, Nathan has analyzed over 400 fin clips across 20 Connecticut streams. From these pinky nail-sized self-regenerating tissue samples,genetic markers called microsatellites amplifyspecific sequences of trout DNA. By comparing these sequences, Nathan and his team can determine how closely-related neighboring fish populations are toone another and from that infer movement rates. To reveal the distribution of genetic information, sampling sites were spaced in 5 kilometer increments within clustered areas. To consider all angles, the team made sure to sample from contrasting systems: the cooler, dendritic (branching) Farmington River basin and the generally linear Shetucket/Thames River basin, to help determine how stream structure may influence the ability to migrate.
The next step will involve overlaying landscape information to explain why some populations have more connections, while others remain isolated. Dam locations, road crossings, and stream temperature all have the potential to hamper the mobility of trout. However, determining which of these barriers are most detrimental to trout mobility will help prioritize conservation initiatives. To determine these priority areas, field information is entered into a mapping program called ArcMap, which plots sample distribution and layers on landscape information. Nathan explains, “By feeding in the genetic information we try to ‘tease out’ how the landscape influences trout movement and determine the cumulative cost of the factors that have the greatest impact.”
Since the start of the project, over 6,000 fin clips have been collected, many still in raw tissue form awaiting DNA extraction. Nathan plans to complete DNA extraction by next winter, reserving his final two years for analyzing the data and “connecting the dots” between the landscape and genetic structure.
“Once I start to utilize more of the landscape modeling, I should be able to break apart why some of these populations are so isolated, while others are connected by varying degrees of trout movement. Initial analysis suggests that obvious barriers such as dams and road-crossings barriers may play a big role, however further investigation will allow us to explore what is going on across different landscape configurations,” Nathan says.
The project will culminate with the drafting of research-supported recommendations to present to state and potentially national officials to improve the future management and conservation of Brook Trout. Based on their findings, Nathan, Vokoun, and their team will pinpoint areas of high importance where fish are prevented from dispersing with the hopes of regaining some of these vital connections. With so many factors impeding migration, prioritizing conservation methods is critical to ensure the most efficient management actions are taken to maximize conservation efforts for this iconic species. While these efforts require some sacrifice, preserving existing routes and forging new paths for these heroic wanderers is imperative for maintaining genetic diversity and ultimately ensuring survival of Brook Trout in the face of a changing climate. So, while it is true that not all those who wander are lost, if things don’t change, the species could be.
Publications resulting from this research:
Kanno, Y., J. C. Vokoun, and B. H. Letcher. 2014. Paired stream-air temperature measurements reveal fine-scale thermal heterogeneity within headwater brook trout stream networks. River Research and Applications 30: 745-755.
Kanno, Y., B. H. Letcher, J. C. Vokoun, and E. F. Zipkin. 2014. Spatial variability in adult brook trout (Salvelinus fontinalis) survival within two intensively surveyed headwater stream networks. Canadian Journal of Fisheries and Aquatic Sciences 71: 1010-1019.
Kanno, Y., J. C. Vokoun, K. E. Holsinger, and B. H. Letcher. 2012. Estimating size-specific brook trout abundance in continuously sampled headwater streams using Bayesian mixed models with zero inflation and overdistpersion. Ecology of Freshwater Fish 21: 404-419.
Kanno, Y., J. C. Vokoun, and B. H. Letcher. 2011. Fine-scale population structure and riverscape genetics of brook trout (Salvelinus fontinalis) distributed continuously along headwater channel networks. Molecular Ecology 20: 3711-3729.
Kanno, Y., J. C. Vokoun, and B. H. Letcher. 2011. Sibship reconstruction for inferring mating systems, dispersal and effective population size in headwater brook trout (Salvelinus fontinalis) populations. Conservation Genetics 12: 619-628.